Very large chimney for 6" flue

[LEW22]

Thanks to help from this forum in my decision making, I finally finished having my Buck Stove 81 installed as an insert in my very large fireplace. I'm attaching a picture so, if you were involved in that discussion you can see the end result (pre-fire). It's hard to tell, but the front projects about 7" into the room so heat radiates from the sides, too. Looks great! Overall, I'm a happy camper.

It's working great today, 45 degrees and no wind, so my house is toasty. But two weeks ago it was 13 degrees and 50 mph wind at my house. Not the norm, but the reason I made the change to an insert in the first place. I'm suspicious that the insert wouldn't have been able to handle it because....

The fireplace the stove is sitting in is a steel box 40"h x 50"w x 30" deep, and the chimney flue is 24" x 24" (yes, 4 sq ft!) ceramic in a 20' tall brick chimney. I understand the original flue area surrounding the new insert's 6" diameter flue is capped up top, but that 6" flue travels up through the original damper of the huge fireplace flue, so the steel fireplace space is basically open to the elements, and heat rises. It takes 15-20 minutes for a strong fire to heat up that fireplace & chimney space. I have a hard time imaging that a huge percentage of the heat generated by this big stove isn't just going up the chimney on a regular basis, as before.

Am I misunderstanding how this works, or is there something I can do to make it more efficient? Can you insulate the some part of the original flue space (maybe at the top?) or somehow close down the unused original damper space? Or is it just what it is? I know I'd have this concern with whatever I had installed, but would welcome suggestions.
Thanks.
--L.

 

[bholler]

Thanks to help from this forum in my decision making, I finally finished having my Buck Stove 81 installed as an insert in my very large fireplace. I'm attaching a picture so, if you were involved in that discussion you can see the end result (pre-fire). It's hard to tell, but the front projects about 7" into the room so heat radiates from the sides, too. Looks great! Overall, I'm a happy camper.

It's working great today, 45 degrees and no wind, so my house is toasty. But two weeks ago it was 13 degrees and 50 mph wind at my house. Not the norm, but the reason I made the change to an insert in the first place. I'm suspicious that the insert wouldn't have been able to handle it because....

The fireplace the stove is sitting in is a steel box 40"h x 50"w x 30" deep, and the chimney flue is 24" x 24" (yes, 4 sq ft!) ceramic in a 20' tall brick chimney. I understand the original flue area surrounding the new insert's 6" diameter flue is capped up top, but that 6" flue travels up through the original damper of the huge fireplace flue, so the steel fireplace space is basically open to the elements, and heat rises. It takes 15-20 minutes for a strong fire to heat up that fireplace & chimney space. I have a hard time imaging that a huge percentage of the heat generated by this big stove isn't just going up the chimney on a regular basis, as before.

Am I misunderstanding how this works, or is there something I can do to make it more efficient? Can you insulate the some part of the original flue space (maybe at the top?) or somehow close down the unused original damper space? Or is it just what it is? I know I'd have this concern with whatever I had installed, but would welcome suggestions.
Thanks.
--L.

View attachment 324489

You need an insulated block off plate.

 

[Solarguy3500]

An easy way to do this is to use foil faced Roxul insulation and install it on the back, sides and top of the fireplace like this:


Make sure you fully tape all the seams with a good quality foil tape.

Also, before cutting any of the pieces make templates out of cardboard so you know they will fit right.

The other way to do this is to make a blockoff plate for the top of the fireplace above the stove using sheet metal, and put unfaced rock wool insulation above it.

 

[mellow]

An easy way to do this is to use foil faced Roxul insulation and install it on the back, sides and top of the fireplace like this:

Where did you find foil faced Roxul? That is awesome that they have come out with that.

 

[Solarguy3500]

Where did you find foil faced Roxul? That is awesome that they have come out with that.

I ordered it from here:

https://www.grainger.com/product/ROXUL-High-Temperature-Insulation-19NE83?bm-verify=AAQAAAAI______LUN5eFaVgHdgh2RkJycBfvglLQzPO4P5NRKisnkl01mWHouS4touZS6rd6eW5N7lLyNV5iQr_WjRf0NvRDV72UjjJy-XfN52lPtf0NzghRDGTP4KzKVOo-zu5yE5EhKLgfQsAtDYTSEGHCLiQP7EXV1XnZKMwRcQ3AmV2HTqV5zlS0RzVvzqJgG7PWHg_qrElvPlGM3bwKbqd_cv60Yo5m9EUoUvfITMRw2kLUKyTHY-x1E-caFh6yEZCl34FsKds7DORnhWq2t2WFxYTpHpEveZtON_KIjYAr8SizdOGB1fAyI05JfAeYfsgs9rCsrKX1XnznSOidSn8shms4ZtEMgVlfMTzhplyaouAtsahmt2FlyooeTOpzmAETH2ywlhFSwPfLHu9RH2tE4xo276jaETORXjoLf6KEIedAYq74fS03RQ9uaVCn7JoJwHnZr0LoN4NOoHjN7Qkeo4LxoeDt6KneUxJvNm10o1GTV-hKX_Ro3s3TiemHpEddnNc_ktdtuMmw1hn3LG6XnXF-m8UsYbMcSQ4bZMlolXZOS5moBZPKKIfbuwGpiIdmwN8zmEfPkDaSI3Vz

I got the 2" thick stuff. They make it in different thicknesses all the way from 1" up to 4". They also have it in unfaced or foil faced.

 

[mellow]

I have used Roxul for many years and one of my pet peeves was that I had to encapsulate it in cement board so the fibers did not get out, this looks like it would solve that issue with the foil facing. Some 3M foil tape should be able to hold up to the temps.

 

[Solarguy3500]

Yeah, the semi-rigid boards of this type of Roxul definitely have binders in them to keep the fibers from getting into the air and the foil facing makes sure of that too. Using foil tape (make sure it's the good quality stuff that's rated for high temps and not the cheap stuff) also helps with air sealing the gaps between pieces of Roxul.